Hornwort
Ceratophyllum demersum
Also known asCoontail
Water parameters
Light and nutrients
Substrate type: any. Propagation: fragmentation.
Substrate compatibility
| Substrate | pH effect | Nutrient load |
|---|---|---|
| Bare bottom (no substrate) (Bare bottom) | n/a | none |
| Inert sand (Pool filter sand) | neutral / inert | none |
| Inert gravel (Aquarium gravel) | neutral / inert | none |
| Limestone gravel (Crushed coral) | raises pH | none |
| Aquasoil (ADA Amazonia) | lowers pH | very high |
| Mineralized clay substrate (Seachem Fluorite) | neutral / inert | moderate |
| Dirted tank (mineralized topsoil) (DIY soil substrate) | slightly acidic | very high |
| Wood and rock mounts (Hardscape mount) | varies | none |
With fish
Origin and habitat
Distributed across every continent except Antarctica. The species (Ceratophyllum demersum) is one of the most widespread aquatic plants in the world, found in ponds, lakes, slow rivers, and ditches from subarctic to tropical regions. Hornwort is a rootless plant: it floats freely in the water column or tangles loosely in substrate and other plants. The stems are covered in whorls of dark green, forked, needle-like leaves that give the plant a coarse, bristly texture. Stems can reach 2 m in the wild. The plant is a major component of natural freshwater ecosystems, providing shelter for fish fry, invertebrates, and aquatic insects. In the aquarium trade, hornwort is sold as a fast-growing, nearly indestructible oxygenating plant. It's also one of the most effective nutrient sponges among commonly available aquarium plants.
Outdoor pond use
- USDA zones
- 3–13 (winter low around -40°C or warmer)
Care notes
Virtually indestructible. Grows in any light (low to high), any temperature (10–30°C), any water chemistry (pH 6.0-8.5, soft to very hard water), and without CO2. Growth is fast: under moderate light, stems elongate 5–15 cm per week. Under high light with nutrients, growth is aggressive and requires frequent thinning. The plant can be weighted and planted in substrate (where it doesn't root but stays anchored by gravity) or left floating. Floating hornwort grows fastest and provides excellent shade and fry cover. The main drawback is needle drop: hornwort sheds its fine leaves when conditions change (especially temperature fluctuations or low light), creating a mess of tiny green needles that clog filters and coat the substrate. This shedding is temporary during acclimation but can be chronic in marginal conditions. Hornwort is allelopathic: it releases chemicals that inhibit the growth of some algae and competing plants, which is an advantage for algae control but can affect nearby sensitive species. Propagation is by fragmentation; any stem piece with a few whorls of leaves grows into a new plant. In aquaponics, hornwort is a useful floating nutrient filter that's nearly impossible to kill. Trim excess growth aggressively and often.